There are known porous coatings referred to as TUFI (Toughened Unipiece Fibrous Insulation) used for thermal insulation and which comprise high silica borosilicate (77.5 weight %) based RCG glass (Reaction Cured Glass; research conducted by the NASA Research Center), silicon tetraboride (2.5 weight %) and molybdenum disilicide ((20 weight %) (D. B. Laiser et al., Advanced Porous Coating for Low-Density Ceramic Insulation Materials, J. Amer. Ceram. Soc., vol. 72, No. 6, pages 1003-1010, 1989). The ceramic substrate used is an AETB (Alumina Enhanced Thermal Barrier) type material comprising amorphous silica (SiO.sub.2) fibers (40 to 85 weight %), aluminoborosilicate fibers (10 weight %) and alumina fibers (5 to 50 weight %). The coating is prepared using slurry coating-firing techniques, including fine milling of the said components, blending them with ethanol as carrier, applying the slurry thus obtained to the surface of the substrate by spraying using a spray gun, and drying and firing the coating at 1220.degree. C.
One drawback of said coating on a ceramic substrate lies in its low operating temperature, namely 1260.degree. C. In a dissociated air flow, at a temperature in excess of 1400.degree. C., bubbles or blisters form therein.
There are also known high alumina ceramic material based coatings comprising 80 to 95 weight % of aluminoborosilicate type glass, the composition (in weight %) of the glass being:
______________________________________ B.sub.2 O.sub.3 17 to 28 Al.sub.2 O.sub.3 3 to 11 SiO.sub.2 balance, ______________________________________
and 5 to 20 weight % of fibrous single crystal aluminum oxide (Al.sub.2 O.sub.3 whiskers) [M. V. Sazonova et al., Coating composition, Inventor's certificate SU 1 331 846 (classification CO3 c 8/24) filed on 21st. Jun. 1985 and published on 23rd. Aug, 1987 (Bulletin No. 31)].
The coating is prepared using slurry coating-firing techniques.
The slurry is prepared using a 50% solution of ethanol in water and is applied to a porous material by brushing with a brush, or by spraying using a spray gun. The coated material is dried at a temperature of 100.degree. to 150.degree. C., and then fired at a temperature of 1300.degree. to 1350.degree. C. for 5 to 15 minutes.
One drawback of this coating on a fibrous ceramic substrate is its low erosion resistance. In an air flow at a temperature of 1300.degree. to 1350.degree. C., ablation is 0.2 to 0.4 g/m.sup.2.
To form coatings having a higher operating temperature, it is necessary to use more refractory oxide based materials. At the same time, it is necessary to ensure the absence of shrinkage and distortion of the fibrous ceramic substrate during formation of the coating.
These problems can be overcome by using a sol-gel technology which makes it possible to develop protective coatings based on refractory oxides such as SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2, etc. on glass, ceramic or metal substrates.
The process for producing coatings using the sol-gel technology includes the steps consisting of preparing a solution of organometallic compounds comprising, in particular, alkoxides, which can be subjected to gelation in a controlled manner, applying this solution to a substrate, usually by immersing the substrate in the solution and removing it from the said solution at a perfectly controlled uniform speed, slowly drying the resulting gel to form a porous film and subsequently densifying it by heating at temperatures of 200.degree. to 700.degree. C.
This process makes it possible to obtain a layer with one or more components, which layer imparts specific properties to the surface to be coated.
However, if only solutions of organometallic compounds or sols are used, it is impossible to obtain a continuous or low porosity coating on the surface of a highly porous material.
This problem can be overcome by introducing into the sol a powdery filler which forms the skeleton of the coating.
There is known a coating referred to as "C-9", prepared by the American company Rockwell Intern., Space Transportation Systems Div., for flexible insulation of the AFRSI (Advanced Flexible Reusable Surface Insulation) type, based on silica fibers and comprising 47 weight % of colloidal silica and 53 weight % of ground (powder form) silica.
This coating is prepared by applying to the surface of the fibrous material a mixture of colloidal silica solution or sol (Ludox.RTM. A.S. produced by Du Pont de Nemours) and of amorphous silica powder, to which isopropyl alcohol is added to impart better wettability to the surface of the fibrous material [D. Mui, H. M. Clancy, Development of a Protective Ceramic Coating for Shuttle Orbiter Advanced Flexible Reusable Surface Insulation (AFRSI) , Ceram. Eng. Sci., Proc., vol. 6, Nos. 7-8, pages 793-805, 1985].
The "C-9" coating, which comprises amorphous silica, has a low thermal expansion coefficient but, owing to the presence of silica, it is only compatible with a quartz fiber based substrate. Furthermore, at temperatures above 1000.degree. C., amorphous silica, in the absence of a crystallisation inhibitor such as boron, for example, tends to crystallise with the formation of cristobalite, which leads to reduced heat resistance.